The present application claims priority to Japanese Application No. P2000-096963 filed Mar. 31, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a solid-state imaging device, and particularly to a solid-state imaging device in which a transistor region excluding a light receiving portion and a transfer portion is shielded.
It is known that an MONOS (Metal Oxide Nitride Oxide Semiconductor) gate structure portion in which a gate electrode is formed on a silicon substrate via a gate insulating film composed of a stack of a silicon oxide film, a silicon nitride film, and a silicon oxide film is irradiated with ultraviolet rays, electrons in silicon migrate in the silicon nitride, whereby a threshold voltage Vth of the transistor is shifted.
In a CCD (Charge Coupled Device) solid-state imaging device, each of a light receiving portion and a transfer portion has an MONOS gate structure. The MONOS gate structure is generally shielded with a metal film for suppressing occurrence of the shift of a threshold voltage Vth.
FIG. 4 is a schematic sectional view of a configuration of a portion, extending from an output end of a horizontal transfer register to an output portion, of a related art solid-state imaging device.
Referring to FIG. 4, a gate insulating film 120 having a stack structure of a silicon oxide film 121, a silicon nitride film 122, and a silicon oxide film 123 is formed on a silicon substrate 111. In a horizontal transfer register portion 112 of the solid-state imaging device, first transfer electrodes 131 made from polysilicon and second transfer electrodes 132 made from polysilicon are alternately formed on the gate insulating film 120.
An output portion 113 is formed at the end of the horizontal transfer register portion 112. In this output portion 113, an output gate 141 and a reset gate 142 are formed on the gate insulating film 120. A floating diffusion portion (hereinafter, referred to as xe2x80x9cFD portionxe2x80x9d) 143 is formed in a portion, between the output gate 141 and the reset gate 142, of the silicon substrate 111. A reset drain 144 is formed in a portion, on the side opposed to the FD portion 143 with respect to the reset gate 142, of the silicon substrate 111.
A planarization film 151 is formed on the silicon substrate 111 in such a manner as to cover the above-described elements. A FD electrode 145 connected to the FD portion 143, and a reset drain electrode 146 connected to the reset drain 144 are formed on the planarization film 151. A shield film 155 made from aluminum is formed on the planarization film 151 in such a manner as to be cover the transfer electrodes 131 and 132. A protective film 161 is formed on the planarization film 151 in such a manner as to cover the electrodes 145 and 146 and the shield film 155.
In the CCD solid-state imaging device having the above configuration, the output gate 141 as the final transfer portion of the horizontal register and the reset gate 142 are not covered with the shield film. For a CCD solid-state imaging device used for a video camera or an electronic still camera, however, there does not occur a problem associated with the shift of a threshold voltage Vth due to irradiation of ultraviolet rays because the protective film 161 absorbs the ultraviolet rays.
However, for a CCD solid-state imaging device of a type of giving a sensitivity in an ultraviolet region having a wavelength of, for example, 400 nm or less, since the device uses a protective film allowing ultraviolet rays to pass therethrough, the ultraviolet rays enter gates not shielded by a metal film, for example, an output gate and a reset gate. When ultraviolet rays enter an output gate and a reset gate, a threshold voltage Vth is shifted, to cause a problem that the transfer and reset of electric charges cannot be performed.
An object of the present invention is to provide a solid-state imaging device capable of preventing ultraviolet rays from entering a region of an MONOS gate structure, excluding a light receiving portion and a transfer portion, of the solid-state imaging device, thereby preventing the shift of a threshold voltage Vth and enhancing the reliability of the transfer or reset of electric charges.
To achieve the above object, according to an aspect of the present invention, there is provided a solid-state imaging device having a gate structure including an oxide film and a nitride film, including: upper layer films allowing ultraviolet rays having a wavelength of 400 nm or less to pass therethrough; and a metal made shield film formed in such a manner as to cover a region of the gate structure including an oxide film and a nitride film, excluding a light receiving portion and a transfer portion, of the solid-state imaging device.
According to another aspect of the present invention, there is provided a solid-state imaging device having a gate structure including an oxide film and a nitride film, including: upper layer films allowing ultraviolet rays having a wavelength of 400 nm or less to pass therethrough; and an organic film capable of absorbing the ultraviolet rays, the organic film being formed in such a manner as to cover a region of the gate structure including an oxide film and a nitride film, excluding a light receiving portion and a transfer portion, of the solid-state imaging device.
With these configurations, since the metal made shield film or the organic film capable of absorbing ultraviolet rays is provided to cover an MONOS gate structure region, excluding a light receiving portion and a transfer portion, of the solid-state imaging device, it is possible to prevent ultraviolet rays from entering the MONOS gate structure excluding the light receiving portion and transfer portion, for example, an output gate and a reset gate. As a result, it is possible to prevent the deterioration of the solid-state imaging device and hence to improve the reliability of the solid-state imaging device.